The co-ordinated operation - one of the biggest mounted in Northern Ireland - specifically targeted foreign nationals suspected of working illegally on mobile phone accessory stalls.

A spokesman for the Home Office said: "There have been nine arrests so far. Others may follow. Investigations are ongoing."

Home Office criminal investigation officers swooped on phone carts in three shopping centres in Belfast, two in Derry as well as others in Armagh, Lisburn, Ballymena and Enniskillen, Dungannon and Bangor earlier today.

It is understood most of those arrested were from India and Pakistan.

The suspects were taken to a detention centre in Larne, Co Antrim, where they could be held for up to five days awaiting deportation.

Carolyne Lindsay, from the Home Office, said: "Foreign nationals who are found working illegally in this country should be in no doubt that they will be caught, arrested and removed from the UK.

"We are determined to crack down on immigration offenders and those who exploit them. Anyone found living or working here illegally is liable to be detained and removed."

Investigations are also being carried out to establish the identity of the employers.

In a statement, the Home Office said every year civil penalties were imposed on hundreds of companies which failed to carry out legally required checks on their staff.

The Home Office has trebled its efforts in tackling illegal immigration in Northern Ireland over the past year.

The organisation's deputy director in the region, Mike Golding, said public tip-offs had helped their investigations.

He said: "Criminal investigators have been working on information that we received that people who have no right to work in the UK were operating mobile phone carts in different shopping malls across Northern Ireland.

"That information proved right with nine people being detained today."

Mr Golding also appealed for more people to come forward with more information.

He told the BBC: "I hope it will help the public to help us more in reporting people who are suspected of working illegally because it is just not fair. It undercuts legitimate business, undercuts people who want to work within the law in Northern Ireland and it undermines the local economy."